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rfc:rfc3682

Network Working Group V. Gill Request for Comments: 3682 J. Heasley Category: Experimental D. Meyer

                                                         February 2004
           The Generalized TTL Security Mechanism (GTSM)

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

 The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6)
 to protect a protocol stack from CPU-utilization based attacks has
 been proposed in many settings (see for example, RFC 2461).  This
 document generalizes these techniques for use by other protocols such
 as BGP (RFC 1771), Multicast Source Discovery Protocol (MSDP),
 Bidirectional Forwarding Detection, and Label Distribution Protocol
 (LDP) (RFC 3036).  While the Generalized TTL Security Mechanism
 (GTSM) is most effective in protecting directly connected protocol
 peers, it can also provide a lower level of protection to multi-hop
 sessions.  GTSM is not directly applicable to protocols employing
 flooding mechanisms (e.g., multicast), and use of multi-hop GTSM
 should be considered on a case-by-case basis.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Assumptions Underlying GTSM. . . . . . . . . . . . . . . . . .  2
     2.1.  GTSM Negotiation . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Assumptions on Attack Sophistication . . . . . . . . . .  3
 3.  GTSM Procedure . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Multi-hop Scenarios. . . . . . . . . . . . . . . . . . .  4
           3.1.1.  Intra-domain Protocol Handling . . . . . . . . .  5
 4.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .  5
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . .  5
     5.1.  TTL (Hop Limit) Spoofing . . . . . . . . . . . . . . . .  5
     5.2.  Tunneled Packets . . . . . . . . . . . . . . . . . . . .  6
           5.2.1.  IP in IP . . . . . . . . . . . . . . . . . . . .  6

Gill, et al. Experimental [Page 1] RFC 3682 Generalized TTL Security Mechanism February 2004

           5.2.2.  IP in MPLS . . . . . . . . . . . . . . . . . . .  7
     5.3.  Multi-Hop Protocol Sessions. . . . . . . . . . . . . . .  8
 6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .  8
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Normative References . . . . . . . . . . . . . . . . . .  8
     7.2.  Informative References . . . . . . . . . . . . . . . . .  9
 8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
 9.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11

1. Introduction

 The Generalized TTL Security Mechanism (GTSM) is designed to protect
 a router's TCP/IP based control plane from CPU-utilization based
 attacks.  In particular, while cryptographic techniques can protect
 the router-based infrastructure (e.g., BGP [RFC1771], [RFC1772]) from
 a wide variety of attacks, many attacks based on CPU overload can be
 prevented by the simple mechanism described in this document.  Note
 that the same technique protects against other scarce-resource
 attacks involving a router's CPU, such as attacks against
 processor-line card bandwidth.
 GTSM is based on the fact that the vast majority of protocol peerings
 are established between routers that are adjacent [PEERING].  Thus
 most protocol peerings are either directly between connected
 interfaces or at the worst case, are between loopback and loopback,
 with static routes to loopbacks.  Since TTL spoofing is considered
 nearly impossible, a mechanism based on an expected TTL value can
 provide a simple and reasonably robust defense from infrastructure
 attacks based on forged protocol packets.
 Finally, the GTSM mechanism is equally applicable to both TTL (IPv4)
 and Hop Limit (IPv6), and from the perspective of GTSM, TTL and Hop
 Limit have identical semantics.  As a result, in the remainder of
 this document the term "TTL" is used to refer to both TTL or Hop
 Limit (as appropriate).
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119].

2. Assumptions Underlying GTSM

 GTSM is predicated upon the following assumptions:
 (i)    The vast majority of protocol peerings are between adjacent
        routers [PEERING].

Gill, et al. Experimental [Page 2] RFC 3682 Generalized TTL Security Mechanism February 2004

 (ii)   It is common practice for many service providers to ingress
        filter (deny) packets that have the provider's loopback
        addresses as the source IP address.
 (iii)  Use of GTSM is OPTIONAL, and can be configured on a per-peer
        (group) basis.
 (iv)   The router supports a method of classifying traffic destined
        for the route processor into interesting/control and
        not-control queues.
 (iv)   The peer routers both implement GTSM.

2.1. GTSM Negotiation

 This document assumes that GTSM will be manually configured between
 protocol peers.  That is, no automatic GTSM capability negotiation,
 such as is envisioned by RFC 2842 [RFC2842] is assumed or defined.

2.2. Assumptions on Attack Sophistication

 Throughout this document, we assume that potential attackers have
 evolved in both sophistication and access to the point that they can
 send control traffic to a protocol session, and that this traffic
 appears to be valid control traffic (i.e., has the source/destination
 of configured peer routers).
 We also assume that each router in the path between the attacker and
 the victim protocol speaker decrements TTL properly (clearly, if
 either the path or the adjacent peer is compromised, then there are
 worse problems to worry about).
 Since the vast majority of our peerings are between adjacent routers,
 we can set the TTL on the protocol packets to 255 (the maximum
 possible for IP) and then reject any protocol packets that come in
 from configured peers which do NOT have an inbound TTL of 255.
 GTSM can be disabled for applications such as route-servers and other
 large diameter multi-hop peerings.  In the event that an the attack
 comes in from a compromised multi-hop peering, that peering can be
 shut down (a method to reduce exposure to multi-hop attacks is
 outlined below).

3. GTSM Procedure

 GTSM SHOULD NOT be enabled by default.  The following process
 describes the per-peer behavior:

Gill, et al. Experimental [Page 3] RFC 3682 Generalized TTL Security Mechanism February 2004

  (i)   If GTSM is enabled, an implementation performs the following
        procedure:
        (a)  For directly connected routers,
            o Set the outbound TTL for the protocol connection to 255.
            o For each configured protocol peer:
              Update the receive path Access Control List (ACL) or
              firewall to only allow protocol packets to pass onto the
              Route Processor (RP) that have the correct <source,
              destination, TTL> tuple.  The TTL must either be 255
              (for a directly connected peer), or 255-(configured-
              range-of-acceptable-hops) for a multi-hop peer.  We
              specify a range here to achieve some robustness to
              changes in topology.  Any directly connected check MUST
              be disabled for such peerings.
              It is assumed that a receive path ACL is an ACL that is
              designed to control which packets are allowed to go to
              the RP.  This procedure will only allow protocol packets
              from adjacent router to pass onto the RP.
        (b)  If the inbound TTL is 255 (for a directly connected
             peer), or 255-(configured-range-of-acceptable-hops) (for
             multi-hop peers), the packet is NOT processed.  Rather,
             the packet is placed into a low priority queue, and
             subsequently logged and/or silently discarded.  In this
             case, an ICMP message MUST NOT be generated.
  (ii)  If GTSM is not enabled, normal protocol behavior is followed.

3.1. Multi-hop Scenarios

 When a multi-hop protocol session is required, we set the expected
 TTL value to be 255-(configured-range-of-acceptable-hops).  This
 approach provides a qualitatively lower degree of security for the
 protocol implementing GTSM (i.e., a DoS attack could theoretically be
 launched by compromising some box in the path).  However, GTSM will
 still catch the vast majority of observed DDoS attacks against a
 given protocol.  Note that since the number of hops can change
 rapidly in real network situations, it is considered that GTSM may
 not be able to handle this scenario adequately and an implementation
 MAY provide OPTIONAL support.

Gill, et al. Experimental [Page 4] RFC 3682 Generalized TTL Security Mechanism February 2004

3.1.1. Intra-domain Protocol Handling

 In general, GTSM is not used for intra-domain protocol peers or
 adjacencies.  The special case of iBGP peers can be protected by
 filtering at the network edge for any packet that has a source
 address of one of the loopback addresses used for the intra-domain
 peering.  In addition, the current best practice is to further
 protect such peers or adjacencies with an MD5 signature [RFC2385].

4. Acknowledgments

 The use of the TTL field to protect BGP originated with many
 different people, including Paul Traina and Jon Stewart.  Ryan
 McDowell also suggested a similar idea.  Steve Bellovin, Jay
 Borkenhagen, Randy Bush, Vern Paxon, Pekka Savola, and Robert Raszuk
 also provided useful feedback on earlier versions of this document.
 David Ward provided insight on the generalization of the original
 BGP-specific idea.

5. Security Considerations

 GTSM is a simple procedure that protects single hop protocol
 sessions, except in those cases in which the peer has been
 compromised.

5.1. TTL (Hop Limit) Spoofing

 The approach described here is based on the observation that a TTL
 (or Hop Limit) value of 255 is non-trivial to spoof, since as the
 packet passes through routers towards the destination, the TTL is
 decremented by one.  As a result, when a router receives a packet, it
 may not be able to determine if the packet's IP address is valid, but
 it can determine how many router hops away it is (again, assuming
 none of the routers in the path are compromised in such a way that
 they would reset the packet's TTL).
 Note, however, that while engineering a packet's TTL such that it has
 a particular value when sourced from an arbitrary location is
 difficult (but not impossible), engineering a TTL value of 255 from
 non-directly connected locations is not possible (again, assuming
 none of the directly connected neighbors are compromised, the packet
 hasn't been tunneled to the decapsulator, and the intervening routers
 are operating in accordance with RFC 791 [RFC791]).

Gill, et al. Experimental [Page 5] RFC 3682 Generalized TTL Security Mechanism February 2004

5.2. Tunneled Packets

 An exception to the observation that a packet with TTL of 255 is
 difficult to spoof occurs when a protocol packet is tunneled to a
 decapsulator who then forwards the packet to a directly connected
 protocol peer.  In this case the decapsulator (tunnel endpoint) can
 either be the penultimate hop, or the last hop itself.  A related
 case arises when the protocol packet is tunneled directly to the
 protocol peer (the protocol peer is the decapsulator).
 When the protocol packet is encapsulated in IP, it is possible to
 spoof the TTL.  It may also be impossible to legitimately get the
 packet to the protocol peer with a TTL of 255, as in the IP in MPLS
 cases described below.
 Finally, note that the security of any tunneling technique depends
 heavily on authentication at the tunnel endpoints, as well as how the
 tunneled packets are protected in flight.  Such mechanisms are,
 however, beyond the scope of this memo.

5.2.1. IP in IP

 Protocol packets may be tunneled over IP directly to a protocol peer,
 or to a decapsulator (tunnel endpoint) that then forwards the packet
 to a directly connected protocol peer (e.g., in IP-in-IP [RFC2003],
 GRE [RFC2784], or various forms of IPv6-in-IPv4 [RFC2893]).  These
 cases are depicted below.
  Peer router ---------- Tunnel endpoint router and peer
   TTL=255     [tunnel]   [TTL=255 at ingress]
                          [TTL=255 at egress]
  Peer router ---------- Tunnel endpoint router ----- On-link peer
   TTL=255     [tunnel]   [TTL=255 at ingress]  [TTL=254 at ingress]
                          [TTL=254 at egress]
 In the first case, in which the encapsulated packet is tunneled
 directly to the protocol peer, the encapsulated packet's TTL can be
 set arbitrary value.  In the second case, in which the encapsulated
 packet is tunneled to a decapsulator (tunnel endpoint) which then
 forwards it to a directly connected protocol peer, RFC 2003 specifies
 the following behavior:
    When encapsulating a datagram, the TTL in the inner IP header is
    decremented by one if the tunneling is being done as part of
    forwarding the datagram; otherwise, the inner header TTL is not
    changed during encapsulation.  If the resulting TTL in the inner
    IP header is 0, the datagram is discarded and an ICMP Time

Gill, et al. Experimental [Page 6] RFC 3682 Generalized TTL Security Mechanism February 2004

    Exceeded message SHOULD be returned to the sender.  An
    encapsulator MUST NOT encapsulate a datagram with TTL = 0.
 Hence the inner IP packet header's TTL, as seen by the decapsulator,
 can be set to an arbitrary value (in particular, 255).  As a result,
 it may not be possible to deliver the protocol packet to the peer
 with a TTL of 255.

5.2.2. IP in MPLS

 Protocol packets may also be tunneled over MPLS to a protocol peer
 which either the penultimate hop (when the penultimate hop popping
 (PHP) is employed [RFC3032]), or one hop beyond the penultimate hop.
 These cases are depicted below.
  Peer router ---------- Penultimate Hop (PH) and peer
   TTL=255     [tunnel]   [TTL=255 at ingress]
                          [TTL<=254 at egress]
  Peer router ---------- Penultimate Hop  -------- On-link peer
   TTL=255     [tunnel]   [TTL=255 at ingress]  [TTL <=254 at ingress]
                          [TTL<=254 at egress]
 TTL handling for these cases is described in RFC 3032.  RFC 3032
 states that when the IP packet is first labeled:
    ... the TTL field of the label stack entry MUST BE set to the
    value of the IP TTL field.  (If the IP TTL field needs to be
    decremented, as part of the IP processing, it is assumed that
    this has already been done.)
 When the label is popped:
    When a label is popped, and the resulting label stack is empty,
    then the value of the IP TTL field SHOULD BE replaced with the
    outgoing TTL value, as defined above.  In IPv4 this also requires
    modification of the IP header checksum.
 where the definition of "outgoing TTL" is:
    The "incoming TTL" of a labeled packet is defined to be the value
    of the TTL field of the top label stack entry when the packet is
    received.

Gill, et al. Experimental [Page 7] RFC 3682 Generalized TTL Security Mechanism February 2004

 The "outgoing TTL" of a labeled packet is defined to be the larger
 of:
    a) one less than the incoming TTL,
    b) zero.
 In either of these cases, the minimum value by which the TTL could be
 decremented would be one (the network operator prefers to hide its
 infrastructure by decrementing the TTL by the minimum number of LSP
 hops, one, rather than decrementing the TTL as it traverses its MPLS
 domain).  As a result, the maximum TTL value at egress from the MPLS
 cloud is 254 (255-1), and as a result the check described in section
 3 will fail.

5.3. Multi-Hop Protocol Sessions

 While the GTSM method is less effective for multi-hop protocol
 sessions, it does close the window on several forms of attack.
 However, in the multi-hop scenario GTSM is an OPTIONAL extension.
 Protection of the protocol infrastructure beyond what is provided by
 the GTSM method will likely require cryptographic machinery such as
 is envisioned by Secure BGP (S-BGP) [SBGP1,SBGP2], and/or other
 extensions.  Finally, note that in the multi-hop case described
 above, we specify a range of acceptable TTLs in order to achieve some
 robustness to topology changes.  This robustness to topological
 change comes at the cost of the loss of some robustness to different
 forms of attack.

6. IANA Considerations

 This document creates no new requirements on IANA namespaces
 [RFC2434].

7. References

7.1. Normative References

 [RFC791]   Postel, J., "Internet Protocol Specification", STD 5, RFC
            791, September 1981.
 [RFC1771]  Rekhter, Y. and T. Li (Editors), "A Border Gateway
            Protocol (BGP-4)", RFC 1771, March 1995.
 [RFC1772]  Rekhter, Y. and P. Gross, "Application of the Border
            Gateway Protocol in the Internet", RFC 1772, March 1995.
 [RFC2003]  Perkins, C., "IP Encapsulation with IP", RFC 2003, October
            1996.

Gill, et al. Experimental [Page 8] RFC 3682 Generalized TTL Security Mechanism February 2004

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
            Signature Option", RFC 2385, August 1998.
 [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
            Discover for IP Version 6 (IPv6)", RFC 2461, December
            1998.
 [RFC2784]  Farinacci, D., "Generic Routing Encapsulation (GRE)", RFC
            2784, March 2000.
 [RFC2842]  Chandra, R. and J. Scudder, "Capabilities Advertisement
            with BGP-4", RFC 2842, May 2000.
 [RFC2893]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
            IPv6 Hosts and Routers", RFC 2893, August 2000.
 [RFC3032]  Rosen, E. Tappan, D., Fedorkow, G., Rekhter, Y.,
            Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack
            Encoding", RFC 3032, January 2001.
 [RFC3036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
            B. Thomas, "LDP Specification", RFC 3036, January 2001.
 [RFC3392]  Chandra, R. and J. Scudder, "Capabilities Advertisement
            with BGP-4", RFC 3392, November 2002.
 [SBGP1]    Kent, S., C. Lynn, and K. Seo, "Secure Border Gateway
            Protocol (Secure-BGP)", IEEE Journal on Selected Areas in
            Communications, volume 18, number 4, April, 2000.
 [SBGP2]    Kent, S., C. Lynn, J. Mikkelson, and K. Seo, "Secure
            Border Gateway Protocol (S-BGP) -- Real World Performance
            and Deployment Issues", Proceedings of the IEEE Network
            and Distributed System Security Symposium, February, 2000.

7.2. Informative References

 [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding
            Detection", Work in Progress, June 2003.
 [PEERING]  Empirical data gathered from the Sprint and AOL backbones,
            October, 2002.

Gill, et al. Experimental [Page 9] RFC 3682 Generalized TTL Security Mechanism February 2004

 [RFC2028]  Hovey, R. and S. Bradner, "The Organizations Involved in
            the IETF Standards Process", BCP 11, RFC 2028, October
            1996.
 [RFC2434]  Narten, T., and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 [RFC3618]  Meyer, D. and W. Fenner, Eds., "The Multicast Source
            Discovery Protocol (MSDP)", RFC 3618, October 2003.

8. Authors' Addresses

 Vijay Gill
 EMail: vijay@umbc.edu
 John Heasley
 EMail: heas@shrubbery.net
 David Meyer
 EMail: dmm@1-4-5.net

Gill, et al. Experimental [Page 10] RFC 3682 Generalized TTL Security Mechanism February 2004

9. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Gill, et al. Experimental [Page 11]

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